The present invention relates to a progammable reference voltage circuit, and more particularly to a programmable reference voltage circuit capable of setting an output voltage level in accordance with exeternal data.
A conventional programmable reference voltage circuit will be described with reference to FIG. 1. The conventional programmable reference voltage circuit has three transistors Q1, Q2 and Q3 and a current source 4. The transistor Q1 has a base and a collector which are interconnected to each other and also connected through a resistor R1 to a reference voltage output terminal 1 on which a reference voltage output Vref appears. A current I1 flows through the resistor R1 from the reference voltage output terminal 1 to the base and collector of the transistor Q1. The transistor Q1 has an emitter connected to a ground terminal 2 kept to have a ground potential. The transitor Q2 has a collector which is connected through a resistor R2 to the reference voltage output terminal 1. A current I2 flows through the resistor R2 from the reference voltage output terminal 1 to the collector of the transistor Q2. The transistor Q2 has an emitter connected through a resistor R3 to the ground terminal 2, and a base connected to the base of the transistor Q1. The transistor Q3 has a collector which is connected to the reference voltage output terminal 1 and a base connected to the collector of the transistor Q2 as well as an emitter connected to the ground terminal 2. A power voltage terminal 3 is connected through the current source 4 to the collector of the transistor Q3 and to the reference voltage output terminal 1 as well as to the resistors R1 and R2. An emitter size ratio of the transistor Q1 to the transistor Q2 is 1: N. An emitter size ratio of the transistor Q1 to the transistor Q3 is 1:1.
The output voltage Vref appearing on the reference voltage output terminal 1 is defined by the emitter size ratio of transistors Q1 and Q2 and the three resistors R1, R2 and R3. The output voltage Vref may be expressed by the following equation, Vbe means a base-emitter voltage. V.sub.BEQ1 means the base-mitter voltage of the transistor Q1. EQU Vref=I2.times.R2+V.sub.BEQ3 (1)
The current I2 will be found. EQU V.sub.BEQ1 =V.sub.BEQ2 +R3.times.I2 PA1 The above equation may be rewritten to EQU R3.times.I2=V.sub.BEQ1 -V.sub.BEQ2 (2) EQU V.sub.BEQ1 =(kT/q)ln(I1/Is) (3) EQU V.sub.BEQ2 =(kT/g)ln)I2/N-Is) (4) EQU I1=(Vref-V.sub.BEQ1)/R1 (5) EQU I2=(Vref-V.sub.BEQ3)/R2 (6)
where "k" is the Boltzmann's constant, "T" is the absolute temperature "q" is the charge of electron and "Is" is the saturation current. The equations (3), (4), (5), and (6) are substituted for the equation (2) to obtain the following equation. EQU I2.multidot.R3=(kT)/q[ln{(Vref-V.sub.BEQ1)/(Is.multidot.R1)ln {(Vref-V.sub.BEQ3)/(N.multidot.Is.multidot.R1)}]
Since the transistors Q1 and Q3 have the same emitter size, it is possible to approximate as follows. EQU V.sub.BEQ1 =V.sub.BEQ3 (7)
The equation (8) is substituted for the equation (1) to obtain the following equation. EQU Vref=V.sub.BEQ3 +(R2/R3).multidot.(kT/q)ln(N.multidot.R2.multidot.R1)
Since a temperature coefficient of the base-emitter voltage V.sub.BEQ3 of the transistor Q3 is -2 mV/.degree. C., the equation is set so that the temperature coefficient in the second term of the right side is 2 mV/.degree. C., whereby zero gradient of the temperature coefficient of the output voltage is obtained. This means that variation of the reference voltage due to temperature variation is eliminated.
The above conventional circuit has a disadvantage in that the reference output voltage "Vref" is fixed or constant. In analog circuits, the reference voltage circuit is applicable to various circuits, for example, a comparator circuit utilizing the reference voltage level as a threshold voltage level a differential amplifier and a constant current circuit suitable for setting the contact current securely. The reference voltage circuit is suitable for setting the bias voltage to be applied to the analog circuit. The bias current is the most basic factor of the analog circuit for setting the consumption current, the bias voltage level, amplification degree, amplitude, and defining a frequency characteristic of the transistor which depends upon the bias current.
It is, however, required or preferable to vary the reference voltage. For example, the threshold voltage level is adjusted to the optimum level. The bias current is varied to obtain the optimum amplification degree or optimum amplitude. Further, it is required to set the optimum frequency characteristic of the transistor.
Since the above conventional reference voltage circuit is capable of generating the predetermined constant reference voltage, many different reference voltage circuits are needed to satisfy the above requirement. The many different reference voltage circuits are included in the integrated circuit and switched by switching elements. This results in an enlargement in size of the integrated circuit and also in an increased manufacturing cost thereof.
Due to the above circumstances, it had been required to develop a reference voltage circuit which is capable of varying an output reference voltage level.